The best way to develop stable software is to identify bugs before they even occur. This is achieved through test-driven development (TDD).
You can never have too many tests.
If you feel like your test suite is getting bloated, shun that feeling – this just means you're doing a good job!
Consider how something could go wrong before you actually start to write the code. Then, turn it into a test.
This is made easy with Django, since every app includes the baked-in tests.py script. SEPS developers should make frequent use of this feature!
With your tests already written, you should now find yourself writing code with the express goal of passing said tests. This should result in far more efficient work, and less time wasted debugging later on.
Schneider, Josh; Smalley, Ian; What is test-driven development (TDD)?